1. Field of the Invention
The present invention relates to a lead frame and a resin sealed semiconductor device using the same, and more particularly to the structure of a resin mold portion of a semiconductor device.
2. Description of the Prior Art
The resin mold portion of a resin sealed semiconductor device is intended to protect semiconductor elements from the external environment and to prevent destruction by external force. The recent requirement for performance of the resin mold portion of a resin sealed semiconductor device must follow up the upgrading of the function of semiconductor elements incorporated therein. The upgrading of the function of semiconductor elements refers to the fact that semiconductor elements have come to have multiple functions. In other words, the functions of circuits implemented by semiconductor elements has advanced, and the circuits have become more complicated. Furthermore, in forming the semiconductor elements, the surface patterns of semiconductor elements have become dimensionally finer and the wiring has been formed in multiple layers. In this way, the surface area of the semiconductor elements has become wider and wider.
On the other hand, the resin mold portion of the resin sealed semiconductor device must respond to the increase of area of the semiconductor elements, and cope with the development of the small and thin resin mold portion as demanded in the market. The resin sealed semiconductor device mounting the semiconductor elements may be roughly classified into the pin insertion type and the surface mount type. The surface mount type semiconductor device is smaller in the thickness of the resin mold portion than the pin insertion type. Accordingly, when the surface mount type semiconductor device is exposed to the environments of high temperature and high humidity, the device's sealing resin absorbs the moisture intruded from the environment by diffusion. In addition, the interior atmosphere of the surface mount type semiconductor device is balanced with the ambient atmosphere in a short time. Therefore the surface mount type semiconductor device which easily absorbed moisture is mounted on the printed circuit board or the like by soldering.
In this soldering process, the semiconductor device is dipped in a solder bath heated to the temperature of about 260 degrees Celsius, or heated by infrared rays to a temperature of about 240 degrees Celsius, or exposed to vapor phase at temperature of about 215 degrees Celsius. There is a difference in the coefficient of thermal expansion between the semiconductor elements mounting die pad and the sealing resin. By heat treatment, hence, delamination occurs in the interface of the semiconductor element mounting die pad the sealing resin which is the material of the resin mold portion. At this time, the absorbed moisture or water invading through suspension leads may be collected in the delamination area. When exposed to the temperature of heat treatment in such a state, it may give rise to destruction of the resin mold portion (package crack through the delamination of interface between the semiconductor element mounting die pad and the resin mold portion). That is, the water content absorbed at heat treatment temperature reaches as high as the saturated steam pressure of water (30 to 40 kg/cm.sup.2). This saturated steam pressure exceeds the mechanical strength of the sealing resin and induces breakdown of the resin mold portion.
The physical breakdown resistance of the resin used in the resin mold portion is proportional to the square of the distance from the lower surface of the semiconductor element mounting die pad to the rear surface of the resin mold portion, that is, the wall thickness of the sealing resin. That is, in order to fabricate the semiconductor device of high physical breakdown resistance sealing resin, the thickness of the sealing resin should be increased. To the contrary, for the surface mount type semiconductor device demanded by the market, the wall thickness of the sealing resin must be reduced. As a result, the surface mount type semiconductor device has a disadvantageous profile for heat treatment when soldering on the substrate.
A conventional surface mount type semiconductor device is shown in FIGS. 6 to 8.
In FIG. 6, a semiconductor element 1 is fabricated, for example, by forming oxide film, interlayer insulation film, polycrystalline silicon film and metal film on a single-crystalline silicon substrate, and forming a fine pattern by ordinary photolithographic technology. In addition, by combining with ordinary diffusion or ion implantation technology, a large capacity memory is fabricated. This semiconductor element 1 is formed in a rectangular parallelogram shape. It is affixed on the top surface of a quadrilateral semiconductor element mounting panel 3 of lead frame 2 by means of adhesive such as silver paste or solder. Bonding pads are disposed, corresponding to lead-out terminals of the semiconductor element mounting die pad 3. The bonding pads and plural leads 4 are connected using metal fine wires 5. The semiconductor element 1 connected thus to the leads 4 is molded with resin for sealing, leaving the front end regions of the leads 4. In this way, the resin sealed semiconductor device is composed.
FIG. 7 shows a sectional view of the semiconductor device shown in FIG. 6.
In this example, the resin mold portion 6 measures 2.7 mm in thickness, 8.89 mm in width, and 17.15 mm in length. The overall dimensions of the semiconductor element mounting die pad 3 are 0.2 mm in thickness, 6.00 mm in width and 15.4 mm in length. The thickness of the leads 4 is 0.2 mm. The thickness as measured from the lower surface of the semiconductor element mounting die pad 3 to the bottom of the resin mold portion 6 is a thin size of 1.05 mm. In this example, in relation to the size of the semiconductor element 1, the surface area of the semiconductor element mounting die pad 3 is 6 mm .times.15.4 mm. That is, the rate of the surface area of the semiconductor element mounting die pad 3 to the area of the resin mold portion 6 is about 61%. In this way, the resin mold portion of the resin sealed semiconductor device is increased in the area of the semiconductor element, and further smaller and thinner resin mold portions are developed.
The semiconductor element mounting die pad 3 is made of a stiff material such as Alloy 42 (iron-nickel alloy). Therefore shrinkage stress occurs by the resin mold portion 6 at the time of resin molding of the semiconductor element 1.
This generation of shrinkage stress is explained in detail by reference to FIG. 8.
FIG. 8 is a magnified view of the region of the semiconductor element 1 and the lead 4 connected with the metal fine wire 5 on the semiconductor element mounting die pad 3.
This shrinkage stress is generated in the central concentrating direction (the direction of arrow A in the diagram) of the semiconductor element mounting die pad 3, in the resin mold portion 6 beneath the semiconductor element mounting die pad 3. A stress branching line B is formed, stretching in the vertical direction from the end portion of the semiconductor element mounting die pad 3.
The semiconductor element mounting die pad 3 is usually formed by punching a thin sheet. Accordingly the cut section is at a right angle to the bottom surface. The stress is concentrated in the stress concentration area C of the resin mold portion 6 contacting with the right-angled end portion of the semiconductor element mounting die pad 3.
In such a thin type resin sealed semiconductor device, cracks due to heat are likely to occur. For example, when quick cooling and quick heating from a temperature of -65 degrees to a temperature of 150 degrees are repeated several times, cracks are formed in the stress concentration area C of the resin mold portion 6. The stress is generated due to a difference of the coefficient of thermal expansion between the semiconductor element mounting die pad 3 and the sealing resin of the resin mold portion 6. Cracks occurring in the stress concentration area C are grown along the stress branching line B. As a result, cracks contacting with the air are formed in the resin mold portion 6. Through these cracks, moisture and impurities contained in the air are introduced. The moisture and impurities finally reach the semiconductor element 1 along the crack.
To solve these problems, a semiconductor element mounting die pad having dimples or slits has been proposed.
FIG. 9 is a cross section of the semiconductor device with dimples, being cut off along the center line of the tie bar.
The dimple structure refers to a structure in which grooves 7 or dimples are formed on the back side of the semiconductor element mounting die pad 3.
Delamination occurs in the interface between the semiconductor element mounting die pad 3 and the sealing resin of the resin mold portion 6. The moisture absorbed in this delamination region and the moisture invading through the tie bar is collected in the delamination area. When heated in such a state, as mentioned above, the resin mold portion 6 is broken. The dimple structure is particularly intended to prevent delamination in the interface of the semiconductor element mounting die pad 3 and the sealing resin of the resin mold portion 6. By forming grooves 7 or dimples in the semiconductor element mounting die pad 3, the contact area with the sealing resin is widened.
FIG. 10 is a cross section of the semiconductor device having slits cut off along the center line of the suspension lead.
FIG. 11 shows a plan view of the semiconductor element mounting panel having slits.
The slit structure refers to a structure in which oval holes 8 are penetrating through the face and back side of the semiconductor element mounting die pad 3.
Slits, as with dimples, are formed to prevent onset of delamination in the interface of the semiconductor element mounting die pad 3 and the sealing resin of the resin mold portion 6.
In the conventional structures, however, a sufficient mechanical strength is not obtained in the thin type surface mounted semiconductor device using the semiconductor element mounting die pad 3 with dimples. That is, when the moisture collected in the delamination area reaches the saturated steam pressure of water by heat treatment, breakdown of the resin mold portion 6 is provoked, exceeding the mechanical strength of the resin.
More specifically, by the grooves 7 or dimples of the semiconductor element mounting die pad 3 with dimples, the contact area with the sealing resin is increased. However, only the resin is injected in the grooves 7 or dimples, and the adhesion strength of the resin and the semiconductor element mounting die pad 3 is not improved greatly.
In the thin type surface mount semiconductor device using the semiconductor element mounting die pad 3 with slits, too, sufficient mechanical strength is not obtained. By the holes 8 in the semiconductor element mounting die pad 3, the contact area with the sealing resin is increased. But only the resin is buried in the holes 8, and the adhesion strength of the resin and semiconductor element mounting die pad 3 is not greatly improved. However, the adhesion strength of slits is greater than that of dimples.
The slits are formed as shown in FIG. 11. When the stress is distributed uniformly in the semiconductor element 1, it is suited to the semiconductor element mounting die pad 3 with slits. However, when the stress distribution is not uniform or the stress varies in the small area in the semiconductor element 1, it is difficult to control the stress.
Furthermore, solder for adhering the semiconductor element 1 does not deposit in the area of the holes 8 of the semiconductor element mounting die pad 3 with slits, and the contact area of the semiconductor element 1 and the semiconductor element mounting die pad 3 decreases. Accordingly, when the moisture collected in the delamination area reaches the saturated steam pressure of water by heat treatment, breakdown of the resin mold portion 6 is induced, exceeding the mechanical strength of the resin.
Thus, in the semiconductor device having dimples or slits, it is difficult to sufficiently prevent breakdown of the resin mold portion 6. If the resin mold portion 6 is broken, or cracks contacting with the air are formed in the resin mold portion 6, moisture and impurities contained in the air invade through the cracks. The moisture and impurities reach the semiconductor element 1 through the cracks. As a result, the adjacent leads 4 are short-circuited by moisture, thereby causing malfunctions.
Or the impurities contained in the sealing resin are extracted by moisture. The extracted impurities may corrode the wiring metal. By this corrosion, the wiring may be disconnected.
In addition, the moisture invading through the cracks gets into the solder or silver paste adhering the semiconductor element 1 on semiconductor element mounting die pad 3. Consequently, the semiconductor element 1 is delaminated off the semiconductor element mounting die pad 3, or the semiconductor element 1 is warped by the subsequent heat treatment. As the semiconductor element 1 is warped, the characteristics of the semiconductor elements formed inside deteriorate.
Therefore, formation of cracks in the semiconductor device will notably impair the reliability of the semiconductor element 1.
In the light of the above problems, it is hence a primary object of the invention to present a highly reliable semiconductor device having a sufficient mechanical strength and capable of controlling the stress in a narrow region, in a thin type surface mount semiconductor device.